The heart of this diy metal detector circuit is the CS209A IC. The metal detector is built with one 100ÁH coil that has 40 mm in diameter and is made of 50 turns/0.4 mm wire. CS209A has one oscillator wich forms a LC circuit, the inductance of the coil will change when it is near metal objects.
LED 1 will light up and the buzzer turns ON when the coil is changing inductance. The setup is easy, VR1 is adjusted (away from any metal objects) so that LED 1 will light and the buzzer sounds on, and then VR1 will be trimmed until led and buzzer are off.